(1) Field of the Invention
The invention relates to high-speed underwater vehicles, and is directed more particularly to supercavitating vehicles which move in a cushion of air underwater.
(2) Description of the Prior Art
Recent investigations into high-speed underwater vehicles have focused attention on providing vehicles which ride a cushion of air to achieve high speeds in water. For a nominal prior art streamlined, fully-wetted underwater vehicle, 70% of the overall drag is skin friction drag; the remainder is pressure or blockage drag. Supercavitation allows for much higher speeds to be sustainable by eliminating, or drastically reducing, skin friction drag at the higher speeds. The conditions for supercavitation require that enough energy be put into the water to vaporize a given volume of water through which an object can travel. This is done by accelerating fluid over a sharp edge, usually the nose of a vehicle, such as a torpedo, so that the pressure drops below the vapor pressure of water. If the speed of the object is not fast enough to travel through the vapor cavity before the cavity collapses, artificial ventilation into the cavity can keep the cavity xe2x80x9copenxe2x80x9d until the object moves past. When a cavity completely encapsulates an object, by vaporous and/or vented cavitation, it is referred to as xe2x80x9csupercavitationxe2x80x9d. The vehicle nose, or xe2x80x9ccavitatorxe2x80x9d, is the only part of the object in constant contact with the water through which the vehicle travels. The cavity closure is positioned behind the vehicle.
When the cavitator and artificial ventilation generate the necessary cavity properties, i.e., sufficient length and diameter of air cushion, it results in a larger air gap between the vehicle and water than is otherwise necessary at the after end of the vehicle. The air, or other selected gas, is drawn through the gap by a propulsion jet plume, and escapes into the ambient water.
It has been found desirable to minimize the downstream entrainment effect of the propulsion plume, to thereby minimize loss of air and to increase life expectancy of a reservoir of ventilation air on-board the vehicle.
An object of the invention is, therefore, to provide a high-speed underwater supercavitating vehicle in which the air cavity at the aft end of the vehicle is reduced while the air cavity otherwise remains appropriately sized and configured for vehicle travel.
With the above and other objects in view, as will hereinafter appear, a feature of the present invention is the provision of a high-speed supercavitating underwater vehicle comprising an elongated hull of circular cross section, the hull having a cavitator at a forward end thereof and means for ventilating gas to form a cavity around the hull in underwater travel, and an expandable annular skirt fixed on the hull and having an outer surface generally contiguous with an outer surface of the hull. The skirt is expandable to increase a diameter of the skirt from slightly above a diameter of the hull to proximate a diameter of the cavity, to define an annular gas film between the expanded skirt and a boundary of the cavity, whereby to substantially reduce the flow of gas from a forward high pressure zone to an after low pressure zone.
The above and other features of the invention, including various novel details of construction and combinations of parts, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular device embodying the invention is shown by way of illustration only and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.